I was at a car show yesterday and spoke to a tuner who told me that the main CAT's just behind the manifolds, where not that restrictive as compared to a straight tube if you were under 620 hp. That seemed completely contrary to every thing I have heard for years. However he said that he had objective data to prove it from 2 cars he tested. I was told that there was less than 3 hp gained with open pipes but a definite sound increase inside the cabin. Those comments made me wonder. Has anyone else ever done a comparison between the stock CAT's vs an open pipe assuming the rest of the exhaust remains the same?